Sound: from noise to communication

H-M. Voipio(1), E. Björk(2), M. Hakumäki(3), T. Nevalainen(4).
(1) Laboratory Animal Centre, University of Oulu, Oulu; (2) Department of Environmental Sciences; (3) Department of Physiology; (4) National Laboratory Animal Center, University of Kuopio, Kuopio, Finland

Animals in laboratory animal facilities are surrounded by sounds. The acoustic environment consists of sounds from equipment controlling environment and from animal care. Furthermore, animals produce sounds themselves by moving or banging the cages and also by vocalising. Continuous white noise sound exposure has been shown to have an effect on animal's physiology and behaviour. In some situations environmental sounds can become harmful to animals. In an animal facility, a wide variety of different sound pitch range from low to very high frequencies occur. Likewise, sound pressure levels vary from low background level, <35 dB (lin), to high levels caused by animal care, for example cage changing, >70-80 dB (lin) or dog barking, 90-100 dB (lin).
In addition to pitch and intensity, type of the sound is important. Fear reactions of rats are stronger to noise type sound than to harmonic or pure tones, and sound structure may be even more significant than sound intensity. In animal units, a typical noise type is caused by running tap water, whereas rat's ultrasonic vocalisation is a tone type sound. While assessing acoustic environment, hearing sensitivity is essential, and differs between species. Many laboratory rodent species hear high frequency sounds, ultrasounds, inaudible to man, but low frequency hearing is poor. Furthermore, hearing ability changes during life, and there are also strain differences. Therefore, when effects of either environmental or experimental sounds are studied, the correct method to account for auditory sensitivity is the use of species specific sound pressure level weighting. More precise spectral analysis should be used to reveal if the sound is inaudible to each of the species within certain frequency range.
Moreover,there are differences in response of animals in different status. The fear reactions to sound of pregnant and mother rats are weaker compared to those of non-pregnant females, while young adult rats are the most sensitive. However, there seems to be no need to provide them with a different acoustic environment. Vocalisation is also a constituent of the sound world. In addition to audible, rodents emit ultrasounds during fighting and reproductive behaviour, between mother and pups and while afraid or in pain. The impact of acoustic environment on vocalisation is unclear, but it has been suggested that ultrasounds can disturb communication. It is, however, important to realise that acoustic environment of a cage is different from that of an animal room, since ultrasounds attenuate even at short distances. The optimal acoustic environment of animals remains unknown. Based on the fear reactions of rats, sudden high frequency noise type, and other sounds at high intensity should be avoided in animal care. As an example, if the rat is removed to clean cage before topping the feedhopper, the sound intensity in the rat's ear is much higher than if the rat would stay in the dirty cage. In some units, whole time background sound has been used to mask those produced by care. Yet, background sound pressure level cannot be raised high enough for this purpose. Therefore, it would be more advisable to change the working routines.
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